1. Field of the Invention
This invention relates to a gas current classifier for classifying a powder by utilizing the Coanda effect. More particularly, the present invention relates to a gas current classifier for classifying a powder into particles with given particle sizes while carrying the powder on air streams and also utilizing the Coanda effect and the differences in inertia force and centrifugal force according to the particle size of each particle of the powder so that a powder containing 50% by number or more of particles with a particle size of 20 .mu.m or smaller can be classified in a good efficiency.
This invention also relates to a process for producing a toner by means of a gas current classifier for classifying a colored resin powder by utilizing the Coanda effect. More particularly, the present invention relates a process for producing a toner for developing electrostatic images, by classifying the powder into colored resin particles with given particle sizes while carrying the colored resin powder on air streams and also utilizing the Coanda effect and the differences in inertia force and centrifugal force according to the particle size of each particle of the powder so that a colored resin powder containing 50% by number or more of particles with a particle size of 20 .mu.m or smaller can be classified in a good efficiency.
2. Related Background Art
For classifying powders, various gas current classifiers are proposed. Among them, there are classifiers making use of rotating blades and classifiers having no moving part. Of these, the classifiers having no moving part include fixed-wall centrifugal classifiers and inertial classifiers. As classifiers utilizing inertia force, Elbow Jet classifiers disclosed, e.g., in Loffier, F. and K. Maly, Symposium on Powder Technology D2 (1981) and commercially available as products by Nittetsu Kogyo, and classifiers disclosed, e.g., in Okuda, S. and Yasukuni, J., Proceedings of International Symposium on Powder Technology '81, 771 (1981) have been proposed as inertial classifiers that can carry out classification within fine-powder range.
In such gas current classifiers, as shown in FIGS. 7 and 8, a powder is jetted into a classifying chamber together with an air stream at a high velocity from a material feed nozzle 16 having an orifice in the classification zone of a classifying chamber 32. In the classifying chamber, a Coanda block 26 is provided and air streams crossing the air stream jetted from the material feed nozzle 16 are introduced, where the powder is separated into a group of coarse powder, a group of median powder and a group of fine powder by the action of centrifugal force produced by the curved air streams flowing along the Coanda block 26 and then classified into the group of coarse powder, the group of median powder and the group of fine powder through means of a classifying wedge 117 and another classifying wedge 118 each having a narrow end that forms a tip.
In such a conventional classifier 101, however, classifying wedge blocks 124 and 125 stand stationary, and the positions of the tips of the classifying wedges 117 and 118, respectively, are adjusted so that the flow rates of the air streams for classification can be correspondingly adjusted, to thereby set the classification points (i.e., the particles sizes at which the powder is classified) to the desired values. Also, the tip positions of the classifying wedges, corresponding to the gravity and given classification points of the powder, are detected and moved to provide control so as to maintain the given flow rates. Such control of only the tip positions of the classifying wedges 117 and 118 tends to cause disturbance of air streams in the vicinity of the tips of wedges, depending on their angles, so that, in some instances, no classification can be carried out with good resulting in unauthorized inclusion of particles of a size which should belong to other group of particles, into the group of particles which originally must have a uniform size. Even when it is desired to change the classification points, the locations of the classifying wedges can not be controlled along the direction of air streams if the tip positions of the classifying wedges are shifted to provide control so as to restore the given flow rates. Not only does it takes time to adjust the classification points to the given values, but also the classification precision becomes low, raising problems to be resolved. In particular, when classification is carried out to produce toners for developing electrostatic images, used in copying machines, printers and so forth, such problems tend to dramatically recur.
In general, toners are required to have various properties. The properties of toners are influenced by starting materials used in toners, and may also be often influenced by processes for producing toners. In the step of classification for producing toners, groups of toner particles which have been classified are required to have sharp particle size distributions, and also it is desired to stably produce good-quality toners at a low cost and with a good efficiency.
As binder resins used in toners, it is common to use resins having a low melting point, a low softening point and a low glass transition point. When a colored resin powder containing such resin is introduced into a classifier to carry out classification, the particles tend to adhere or melt-adhere to the inside of the classifier.
In recent years, as measures for energy saving in copying machines, it has become popular to use soft materials such as wax as binder resins, to make fixing speed higher even in the case of heat fixing, and to use binder resins with a low glass transition point or binder resins with a low softening point so that power consumption necessary for fixing can be decreased and fixing can be carried out at a low temperature.
In addition, in order to improve image quality in copying machines and printers, toner particles have exhibited a gradual tendency to be made finer. In general, as substances become finer, the force acting between particles become larger, and the same applies also to resin particles and toner particles, where the particles more greatly tend to agglomerate as their particle size becomes smaller.
Once an external force such as impact force or frictional force acts on agglomerates of such particles, the particles tend to melt-adhere to the inside of the classifier. In particular, the particles tend to melt-adhere to the tips of classifying wedges. Once such a phenomenon has occurred, the classification precision becomes poor and the classifier is not always operable in a stable state, so that it becomes difficult to stably obtain good-quality classified powders over a long period of time.
From such points of view, it is sought to provide a gas current classifier that can stably and efficiently classify, in particular, colored fine resin powders such as toners in a good precision.